The manufacture of battery plates for lead-acid batteries generally involves a paste mixing, curing and drying operation in which the active materials in the battery paste undergo chemical and physical changes that are used to establish the chemical and physical structure and subsequent mechanical strength necessary to form the battery plate. To produce typical battery plates, materials are added to commercial paste mixing machines in the order of lead oxide, water and sulfuric acid, which are then mixed to a paste consistency. Depending on whether negative or positive plates for the batteries are being produced, conventional additives such as a flock or expander may also be used to modify the properties of the paste and the performance of the plates produced. Other additives may be used to enhance or improve the chemical and physical structure and performance of the battery plates, such as the additive disclosed in U.S. Pat. No. 7,118,830 issued to Boden et al. on Oct. 10, 2006, the entire disclosure of which is herein incorporated by reference.
The negative plates of lead-acid batteries are usually produced by preparing a paste with an expander additive, and then applying this battery paste to electrically conducting lead alloy structures known as grids to produce plates. Typically, these pasted plates are then cured in heated chambers containing air with a high relative humidity. This curing process produces the necessary chemical and physical structure required for subsequent handling and performance in the battery. Following curing, the plates are dried using any suitable means. These plates, comprising negative active material, are then suitable for use in the battery.
The expander, which is usually a mixture of barium sulfate, carbon, and a lignosulfonate or other organic material, is added to the negative plate active material during preparation of the paste. The expander may also incorporate other known ingredients, such as wood flour and soda ash, to improve the performance of the battery. The expander materials can be added separately to the paste during the paste mixing process, but an improved procedure is to mix the constituent materials of the expander before adding them to the paste mix.
The expander performs a number of functions in the negative plate, which will be briefly described. The function of the barium sulfate is to act as a nucleating agent for lead sulfate produced when the plate is discharged.PbPb2++2ePb2++SO42−→PbSO4 The lead sulfate discharge product deposits on the barium sulfate particles assuring homogeneous distribution throughout the active material and preventing coating of the lead particles. The term barium sulfate represents both blanc fixe and barytes forms of this compound and mixtures thereof in particle sizes from 0.5 to 5 micrometers. It is desirable that the barium sulfate crystals have a very small particle size, of the order of 1 micron or less, so that a very large number of small seed crystals are implanted in the negative active material. This ensures that the lead sulfate crystals, which are growing on the barium sulfate nuclei, are small and of a uniform size so that they are easily converted to lead active material when the plate is charged.PbSO4Pb2++SO42−Pb2++2e→Pb
The carbon increases the electrical conductivity of the active material in the discharged state, which improves its charge acceptance. The carbon is usually in the form of carbon black and/or activated carbon. The amount of carbon in the negative active material of conventional expander formulations is only a small fraction of a percent.
The function of the lignosulfonate is more complex. It is chemically adsorbed on the lead active material resulting in a significant increase in its surface area. Without lignosulfonate, the surface area is of the order of approximately 0.2 square meters per gram while, with 0.50% of lignosulfonate, this is increased to approximately 2 square meters per gram. This high surface area increases the efficiency of the electrochemical process which improves the performance of the negative plate. The lignosulfonate also stabilizes the physical structure of the negative active material, which retards degradation during operation of the battery. This property increases the life of the battery in service. The organic material can be any lignosulfonate compound or other suitable organic material that can be adsorbed on the surface of the negative active material and thereby affect its surface area and electrochemical behavior.
Lead-acid batteries are used in a variety of applications including automobiles, industrial motive power, such as for forklift trucks, telecommunications, and standby power systems, i.e., uninterruptible power supply batteries. In addition, the batteries may be of the flooded-electrolyte or valve regulated designs. These require different proportions of the expander components and different addition rates to the active material to give optimum performance and life. Accordingly, expanders can be generally classified according to their application, for example: automotive, industrial motive power and industrial standby power. They may also be subdivided for flooded and valve regulated battery designs. Typical examples of conventional expander formulations for the aforementioned applications are shown in FIG. 1.
A recognized problem with expanders is that the aforementioned conventional expander formulations are not effective, for example, in batteries used in hybrid-electric vehicles. In this application, the battery is operated in a partial-state-of-charge (PSOC) condition which is a condition where it is never fully discharged or fully charged. Under this condition the negative plate always is partially converted to lead sulfate which is not converted to lead by periodic full charging as in conventional batteries. The negative plate also has to be capable of accepting charge at very high rates from current generated during regenerative braking.
It has been shown that, during this type of operation, lead sulfate accumulates on the surface of the negative plate which acts as a barrier to the flow of ions and current necessary to charge the plate. Consequently, the lead sulfate layer progressively increases in thickness resulting in a reduction or degradation in battery performance. Eventually, the performance declines to a point where the battery cannot function properly. A representation of this phenomenon is shown in FIG. 2. This problem has prevented lead-acid batteries from being used in hybrid-electric vehicles.
Consequently, a need exists for improved expander formulations for battery pastes and plates that are effective in overcoming the problem of lead sulfate accumulation during high rate PSOC battery operation, as well as improved expander formulations providing improved battery capacity, efficiency, performance and life for lead-acid batteries of various types. The present disclosure overcomes the above identified disadvantages and/or shortcomings of known prior art expanders, battery pastes and methods for producing negative battery plates, and provides significant improvements there over.